This invention relates to shielding an electrical instrument against electromagnetic interference (EMI), including radio frequency interference (RFI), and electrostatic discharge (ESD), and more particularly to a system of such shielding that minimizes the number of components and their cost while maximizing ease of assembly.
The mass production of low cost instruments at a reasonable profit margin requires a design that permits assembly with a minimum of labor input per unit, a minimum number of parts and a minimum cost for those parts.
In product designs optimized for manufacturability, especially highly automated methods of manufacture, a concept known as "Z-axis assembly" is proving to be very important. Z-axis assembly simply means that a product is assembled by lowering the parts from above onto an existing subassembly. This is especially important in robotic assembly, but many of the same benefits can also be realized even in manual assembly. If automated assembly is being employed, the simplest and most cost effective parts handlers and other robotic machines can be employed to stack and connect parts to an existing sub-assembly quickly and easily if the product has been designed for Z-axis assembly.
Prior art shielding schemes have included electrically conductive plastic housings or separate metal layers connected to the instrument's common (ground) electrical plane. Electrically conductive plastics are expensive and do not provide very effective shielding because they do not really conduct very well. Separate metal layers require some means of both electrical and mechanical attachment. This has traditionally meant the use of additional parts, such as screws or springs, to accomplish that attachment.
What is desired is an adequate EMI, RFI and ESD shielding structure that utilizes a minimum number of inexpensive parts and is compatible with Z-axis insertion techniques.